Effects of Dense Dust Plumes at Enceladus
N. Omidi
Solana Scientific Inc.
Cassini Collaborators:
CAPS: R.L. Tokar
RPWS: D.A. Gurnett, T. Averkamp, W.S. Kurth, W.M. Farrell
MAG :

Cassini Image
Plumes consist of H
2
O group gas + ice (dust)
2

• Interaction with the body of Enceladus ( not important )
• Interaction with neutral gas in the plumes through charge
exchange. Replaces energetic ions with cold ones.
• Interaction with charged dust particles in the plumes
Outstanding Question: How do the two processes compare
& is one dominant over the other?

• 3-D Electromagnetic Hybrid (kinetic ions & dust, fluid electrons) simulations
Investigate the interaction of co-rotating plasma with Enceladus
and plumes of neutral gas and/or dust with a variety of shapes
and sizes.
• Cassini Radio Plasma Wave System (RPWS) dust impact rates
Construct plume models for dust and neutral gas.
• Cassini CAPS Ion Energy Spectrogram
Compare observed flow deceleration and cooling to simulation
results .

Simulation box: 95 x 23 x 190 R
E
, R
E
= 252 km
Enceladus rest frame
Enceladus: Non-conducting, plasma absorbing body
B ~ 320 nT
Co-rotating plasma : O +
N ~ 100 cm -3
V_flow ~ 26 km/s ~ 0.1 V
A
(in Enceladus rest frame)
Ion temp = 35 eV, β = 0.006, V th
Elect temp = 1.3 eV, β = 0.0002
~ 21 km/s

• The interaction between the co-rotating plasma and neutral gas takes
place through charge exchange where a hot, co-rotating ion is replaced with
a cold, nearly stationary ion. This implies change from 10-100s eV to a
few eVs for out gassing speeds 100s m/s to ~1 km/s.
• Use charge exchange cross section of 36 Angstrom 2
• Using the above cross section and local density of neutral gas
(from model) probability of charge exchange is determined.
• Charge exchange is represented by reducing the energy of a co-moving
ion to a few eVs in a random manner (speeds 0-500 m/s).

Upon becoming charged, dust particles interact with the co-rotating plasma through the motional E field.
1.
Dust charging through electron absorption, max
density of charged dust = density of co-rotating
plasma ~100 cm -3 (e.g. Farrell et al., 2009). Used
for modeling E7 encounter
2. Unknown mechanism that results in charged dust
densities of ~>10 4 cm -3 (mostly nano-meter sized
particles). Current conceptual model is generation
of dense plasma (e.g. O + - electron) followed by the
absorption of electrons by dust particles (e.g. Shafiq
et al., 2011; Hill et al., 2012). Used for modeling E3

9

10

E7
11

E7
Low
Sensitivity
High
Sensitivity
Density of micron size dust particle ~10 -6 cm -3 and nano- particles ~ 10 2 cm -3
12

Unified Dust & Neutral Gas Plume Model
Diffused Cone: N
D
= N o
exp (- ψ / ψ o
) / R 2
N
D
= Dust or Neutral Gas Density
N o
= Density at Surface of Enceladus
The angle ψ is measured relative to the Cone-Axis
ψ o specifies the “width” of the plume
The Cone-Axis makes an angle θ with – Z
E
axis
The projection of the Cone-Axis in X
E
– Y
E
plane
makes an angle φ with – X
E
axis
13

E7
14

Plume No.
1
2
5
6
3
4
θ
0
10 o
10 o
30 o
20 o
47 o
φ
0
60 o
25 o
40 o
250 o
250 o
ψ
5 o
10 o
5 o
20 o
20 o
5 o
15

Results of a hybrid run with charged dust and no neutral gas. Plumes
#4, 5 and 6 are used.
Max dust charge density = density of co-rotating plasma
High
Sensitivity
E7
16

Results of a hybrid run with no charged dust & neutral gas with base density ~10 9 cm -3 . Only plumes #5 and 6 are used to match CAPS.
E7
17

During E3 dust densities of
~>10 4 cm -3 have been reported.
RPWS dust impact rates are shown on the right. The top panel also shows the model fit including plumes #4, 5, 6 plus an additional plume and a background component varying by Z.
Panel (b) shows the fit without plumes #4, 5 and 6.
Panel (c) shows contribution of the background component.
E3

Results of a hybrid run with charged dust density of 2x10 4 cm -3 and no neutral gas. Plumes
#5 and 6 are used.
E3

Results of a hybrid run with no charged dust & neutral gas with base density ~10 9 cm -3 .
Plumes #5 and 6 are used.
E3

Results of 5 hybrid runs with neutral gas and varying levels of charged dust.
Max Charged Dust Density
200
2000
5000
10 4
2x10 4 cm -3

CAPS Anode #7
E3
CAPS Anode #4
Results of 2 hybrid runs with no charged dust & neutral gas with base density ~10 9 cm -3 .
Plumes #5 and 6 are used.
Panels (c) and (d) correspond to different f(v) for neutral gas.

• RPWS dust impact rates show more dust activity during E3. The
impact rates are modeled using 3 plumes found for E7 plus 1 additional
plume and dust background.
• Hybrid simulations show good agreement with CAPS interaction region
when we use plumes #5 and 6 similar to E7.
• When using dense dust plumes alone in hybrid simulations the resulting
ion energy spectrograms do not resemble those of CAPS.
• Hybrid simulations with no or small (comparable to background density)
levels of charged dust result in ion energy spectrograms that better
match those observed by CAPS.
• Further addition of dust reduces the level of agreement with CAPS. This
plus problems with generating dense charged dust plumes suggest
densities lower than 10 4 cm -3 .
• Details of ion energy spectrogram sensitive to f(v) of neutral gas.